1. Field of the Invention
The present invention relates to cemented carbide cutting tools having adherent coatings of titanium carbonitride with (W,Ti) C,N type phases present at grain boundaries of the carbonitride, and process for deposition of those coatings.
2. Description of the Related Art
Cemented carbide cutting tools are widely used in metal cutting applications owing to their unique properties of high hardness, toughness, strength, and wear resistance of cemented carbide materials can be further improved by applying a thin layer of a coating of a refractory compound such as titanium carbide, titanium nitride, aluminum oxide, and combinations thereof. Such coatings have widened the application range of cemented carbide tools.
Advances in metal working equipment manufacture and economic necessities of higher productivity have combined to put greater demands of improved performance on the cutting tool materials.
Tungsten carbide based sintered alloys are used as substrate material for the coated cutting tools because of the high mechanical strength, hardness and good wear resistance. As a substrate material, the tungsten carbide based alloy has higher toughness than other materials such as titanium carbide based sintered alloys, titanium nitride based sintered alloys and alumina-based ceramics. However, when the tungsten carbide based substrates are coated with titanium carbide by chemical vapor deposition techniques, a brittle layer of a complex carbide, known as the eta phase, tends to form around the tungsten carbide grains. The brittle layer acts as a site initiating the propagation of cracks so that the cutting inserts with this deleterious complex exhibit a marked reduction in strength.
The deleterious effect of eta phase is described in the papers by Bhat, Cho and Woerner (Relationship between metal-cutting performance and material properties of TiC-coated cemented carbide cutting tools, Surface and Coatings Technology, Vol. 29, 1986, pp. 239-246; the role of interface development during chemical vapor deposition in the performance of TiC-coated cemented carbide cutting tools, Journal of Vacuum Science and Technology, Vol. A4(6), 1986, pp. 2713-2716). 
Another problem encountered during vapor phase deposition is the diffusion of cobalt to the surface and into the TiCN coatings. One technique to avoid these problems is not to use titanium carbide. Several practitioners of the art resort to depositing a film of TiN on the surface first.
Conventional coatings of TiCN on cemented carbides using CVD techniques assume that the substrate surface is uniform and normalized. However, this is not the case. Cemented carbide surfaces normally have irregular sintered surfaces along with surface defects like voids, pores, and loosely held WC crystals on the surface with surface protrusions. In cases where cubic carbide additions are incorporated with the WC matrix, the distribution of the same assumes a random pattern on the surface. As a result, the interface between the coating and the substrate presents a weak area that will result in inferior performance during machining operations. Additional problems include the formation of eta phase at the interface with associated Kirkendall porosity, which weakens the interface further.
As per prior art, conventional CVD deposition of TiCN coating on carbide substrates, presents a problem due to the carbon contribution from the substrate. This is a reason why practitioners of this art deposit a normalizing layer initially consisting of TiC or TiN to stop the substrate contribution of carbon toward the TiCN layer deposition. The present invention allows the deposition of TiCN directly on a cemented carbide substrate without going through techniques of prior art.
The present invention allows a composite wear resistant layer consisting of titanium carbonitride with tungsten present as a network structure delineating the grain boundaries of the carbonitride layer. The technique proposed here is the controlled deposition of a TiCN layer, with an optimum C/N ratio in the gas phase, followed by carbon correction treatment as in U.S. Pat. No. 4,830,886 incorporated herein by reference as if fully set forth. This controlled treatment results in a normalized interface, with the closing of the voids and pores at the interface and elimination of the deleterious eta phase encountered in the prior art. Thus the presence of a controlled TiCN layer with tungsten present at the grain boundaries of the layer providing additional solid solution strengthening for TiCN results in a superior wear resistant coated tool.
The present invention relates to a process for forming a titanium carbonitride layer with tungsten present at the grain boundaries directly on the cemented carbide substrate without the presence of a deleterious eta phase. It is an object of the present invention to eliminate or reduce the presence of the eta phase from the coating/substrate interface region, and to eliminate porosity in the coating layer which is associated with the surface condition of the substrate.
It is a further object to achieve the above without sacrificing the advantageous properties of a titanium carbonitride coating for metal cutting applications.
Further, it is an object of the present invention to enhance one or some of the desirable properties, such as the adhesion of the coating. The present invention is directed to an improved process for obtaining a coated substrate which obviates one or more disadvantages of prior processes.
In accordance with the present invention, there is provided a process for making a cemented carbide cutting insert of the type comprising a substrate of tungsten carbide grains in a cobalt matrix having a layer of titanium carbonitride directly adjacent to said substrate. The substrate directly adjacent said coating comprises tungsten carbide, and cubic carbides of the type (W,Ta, Ti)(C) in a form chemically unreacted with cobalt. More particularly, the substrate directly adjacent to the coating is characterized by the absence of eta phase.
In accordance with the process of the present invention, the undesirable eta phase formed during the vapor phase deposition of TiCN or during the manufacture of the substrate is removed by a carbon correction cycle, which also leads to titanium carbonitride layer composition optimization.